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<li class="navelem"><a class="el" href="../../d5/d10/tutorial_js_root.html">OpenCV.js Tutorials</a></li><li class="navelem"><a class="el" href="../../df/d04/tutorial_js_table_of_contents_gui.html">GUI Features</a></li>  </ul>
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<div class="title">Getting Started with Images </div>  </div>
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<div class="contents">
<div class="textblock"><h2>Goals </h2>
<ul>
<li>Learn how to read an image and how to display it in a web.</li>
</ul>
<h2>Read an image </h2>
<p>OpenCV.js saves images as <a class="el" href="../../d3/d63/classcv_1_1Mat.html" title="n-dimensional dense array class ">cv.Mat</a> type. We use HTML canvas element to transfer <a class="el" href="../../d3/d63/classcv_1_1Mat.html" title="n-dimensional dense array class ">cv.Mat</a> to the web or in reverse. The ImageData interface can represent or set the underlying pixel data of an area of a canvas element.</p>
<dl class="section note"><dt>Note</dt><dd>Please refer to canvas docs for more details.</dd></dl>
<p>First, create an ImageData obj from canvas: </p><div class="fragment"><div class="line">let canvas = document.getElementById(canvasInputId);</div><div class="line">let ctx = canvas.getContext(&#39;2d&#39;);</div><div class="line">let imgData = ctx.getImageData(0, 0, canvas.width, canvas.height);</div></div><!-- fragment --><p>Then, use cv.matFromImageData to construct a <a class="el" href="../../d3/d63/classcv_1_1Mat.html" title="n-dimensional dense array class ">cv.Mat</a>: </p><div class="fragment"><div class="line">let src = cv.matFromImageData(imgData);</div></div><!-- fragment --><dl class="section note"><dt>Note</dt><dd>Because canvas only support 8-bit RGBA image with continuous storage, the <a class="el" href="../../d3/d63/classcv_1_1Mat.html" title="n-dimensional dense array class ">cv.Mat</a> type is cv.CV_8UC4. It is different from native OpenCV because images returned and shown by the native <b>imread</b> and <b>imshow</b> have the channels stored in BGR order.</dd></dl>
<h2>Display an image </h2>
<p>First, convert the type of src to cv.CV_8UC4: </p><div class="fragment"><div class="line">let dst = new cv.Mat();</div><div class="line">// scale and shift are used to map the data to [0, 255].</div><div class="line">src.convertTo(dst, cv.CV_8U, scale, shift);</div><div class="line">// *** is GRAY, RGB, or RGBA, according to src.channels() is 1, 3 or 4.</div><div class="line">cv.cvtColor(dst, dst, cv.COLOR_***2RGBA);</div></div><!-- fragment --><p>Then, new an ImageData obj from dst: </p><div class="fragment"><div class="line">let imgData = new ImageData(new Uint8ClampedArray(dst.data, dst.cols, dst.rows);</div></div><!-- fragment --><p>Finally, display it: </p><div class="fragment"><div class="line">let canvas = document.getElementById(canvasOutputId);</div><div class="line">let ctx = canvas.getContext(&#39;2d&#39;);</div><div class="line">ctx.clearRect(0, 0, canvas.width, canvas.height);</div><div class="line">canvas.width = imgData.width;</div><div class="line">canvas.height = imgData.height;</div><div class="line">ctx.putImageData(imgData, 0, 0);</div></div><!-- fragment --><h2>In OpenCV.js </h2>
<p>OpenCV.js implements image reading and showing using the above method.</p>
<p>We use <b><a class="el" href="../../d4/da8/group__imgcodecs.html#ga288b8b3da0892bd651fce07b3bbd3a56" title="Loads an image from a file. ">cv.imread</a> (imageSource)</b> to read an image from html canvas or img element. </p><dl class="params"><dt>Parameters</dt><dd>
  <table class="params">
    <tr><td class="paramname">imageSource</td><td>canvas element or id, or img element or id. </td></tr>
  </table>
  </dd>
</dl>
<dl class="section return"><dt>Returns</dt><dd>mat with channels stored in RGBA order.</dd></dl>
<p>We use <b><a class="el" href="../../d7/dfc/group__highgui.html#ga453d42fe4cb60e5723281a89973ee563" title="Displays an image in the specified window. ">cv.imshow</a> (canvasSource, mat)</b> to display it. The function may scale the mat, depending on its depth:</p><ul>
<li>If the mat is 8-bit unsigned, it is displayed as is.</li>
<li>If the mat is 16-bit unsigned or 32-bit integer, the pixels are divided by 256. That is, the value range [0,255*256] is mapped to [0,255].</li>
<li>If the mat is 32-bit floating-point, the pixel values are multiplied by 255. That is, the value range [0,1] is mapped to [0,255].</li>
</ul>
<dl class="params"><dt>Parameters</dt><dd>
  <table class="params">
    <tr><td class="paramname">canvasSource</td><td>canvas element or id. </td></tr>
    <tr><td class="paramname">mat</td><td>mat to be shown.</td></tr>
  </table>
  </dd>
</dl>
<p>The above code of image reading and showing could be simplified as below. </p><div class="fragment"><div class="line">let img = cv.imread(imageSource);</div><div class="line">cv.imshow(canvasOutput, img);</div><div class="line">img.delete();</div></div><!-- fragment --><h2>Try it </h2>
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